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Deep Layer Variability in the Eastern North Atlantic the EDYLOC

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Deep Layer Variability in the Eastern North Atlantic the EDYLOC ____________________________________o_c_EA_N_O __ LO_G __ IC_A_A_C_T_A __ 19_8_8_-_v_o_L_._1_1_-_N_o_2~~~----- Current meler array Dynamics Bottom nepheloid layer Deep layer variability Variability Porcupine abyssal plain in the Eastern North Atlantic: Mouillages de courantomètres Dynamique Couche néphéloïde profonde the EDYLOC experiment Variabilité Plaine abyssale Porcupine Annick V ANGRIESHEIM Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Centre de Brest, B. P. 70, 29263 Plouzané, France. Received 7/4/87, in revised form 19/11/87, accepted 24/11/87. ABSTRACT The ÉDYLOC ("Étude DYnamique LOCale") experiment was carried out by IFREMER-Centre de Brest in 1981-1982 in order to describe the dynamics and evaluate the space and time scales of current variability in the deep layer of the Porcupine abyssal plain (North East Atlantic). Six moorings separated by 11 to 73 km were deployed in a flat bottom area near 47°N, 14°30'W. The water depth was 4780 m. The results are described here. A hydrological survey, with nephelometry profiles, shows that a nepheloid layer exists at each station with the strongest signal associated with the only observed bottom mixed layer in the north-western part of the array; it is also there that the currents are the strongest throughout the measurement period. Over the 11 months of record (data return: 96.2%), the horizontally averaged mean speed is 1.26 cmjs at 4000 m and 1.50 cm/s at 10 m above the bottom with a predominancy of the westward-component. The correlation between the two levels is very high, which means that the deepest level dynamics is not influenced by the bottom effect to the extent that one might expect in a bottom boundary layer. A bottom intensification is observed on KM (kinetic energy of the mean flow) and on KE (eddy kinetic energy) and, for both levels, the currents appear more energetic than for other previous experiments. The zero-crossing of the auto-correlation functions is 20 days for the U-component at both levels and 66 and 33 days for the V-component at 4000 and 10 m above the bottom respectively. At both levels, the zero-crossing of the transverse velocity correlation function is 27 km. Oceanol. Acta, 1988, tt, 2, 149-158. RÉSUMÉ Variabilité dans la couche profonde de l'Atlantique Nord-Est : l'expérience ÉDYLOC L'expérience ÉDYLOC (Étude DYnamique LOCale) a eu lieu en 1981-1982 dans le but de décrire la dynamique et évaluer les échelles spatiales et temporelles de la variabilité du courant dans la couche profonde de la plaine abyssale Porcupine dans l'Atlantique Nord-Est. Six mouillages séparés de 11 à 73 km ont été installés dans une région à fond plat autour de 47°N, 14°30'W sur une profondeur d'eau de 4 780 m. Les résultats en sont décrits ici. Une reconnaissance hydrologique, avec des profils de néphélométrie, a montré l'exis­ tence d'une couche néphéloïde à chaque station, la plus intense étant associée à l'unique couche mélangée de fond observée dans la partie nord-ouest de la zone; c'est également à cet endroit que les courants ont été les plus forts pendant toute la période de mesures. Sur les onze mois de mesures (retour des données : 96,2 %), la vitesse moyenne, moyennée horizontalement, est de 1,26 cm/s à 4 000 m et 1,50 cm/s près du fond, avec une prédominance pour la composante Est-Ouest. La corrélation entre les deux niveaux est très élevée, ce qui indique que l'effet de couche limite n'est pas si important. 0399-1784/88/02 149 1 0/$ 3.00/ (Cl Gauthier-Villars 149 A. VANGRIESHEIM Une intensification est observée sur KM, énergie cinétique moyenne et sur KE, énergie cinétique turbulente, et aux deux niveaux, les courants sont plus énergétiques que lors d'expériences précédentes. Le premier passage à zéro de la fonction d'autocorrélation moyenne est à 20 jours aux deux niveaux pour la composante U et à 66 et 33 jours pour la comp-osante V à 4000 rn et au fond, respectivement. Aux deux niveaux, le premier passage à zéro de la fonction de corrélation transverse est 27 km. Oceanol. Acta, 1988, tl, 2, 149-158. INTRODUCTION Another approach to the characterization of the influ­ ence of the bottom in the deep layer is to examine The EDYLOC Experiment ("Étude Dynamique its hydrological fine structure and the existence of a Locale"), was conducted in order to evaluate spatial nepheloid layer. As a result of homogenization due to and temporal scales of deep current variability in the turbulence above the bottom, a bottom mixed layer eastern North Atlantic Porcupine abyssal plain. Long­ (BML) appears in the lower part of the water column, lerm current measurements were carried out with the its thickness depending on the strength of this dynamic deployment of six moorings, with two levels of mea­ activity. It cannot be directly related to the theoretical surements each (4000 and 10 rn above the bottom); a Ekman layer because a BML may be the result of CTD survey was performed during the moorings reco­ superposition of local old and recent BMLs and very cruise. The two level results will allow a compari­ advected BMLs. Armi and Millard (1976) studied the son of the ocean interior dynamics with that of the BMLs over the Hatteras abyssal plain; they found that benthic boundary layer. the BML thickness was correlated with the daily mean speed of the current and its vertical extension was Briefly, the characteristics of the benthic boundary around 6 times the theoretical Ekman layer thickness. layer will be reviewed. Theoretically, in close proximity Nevertheless, the existence of a BML is the signature to the bottom, a "logarithmic layer" must be found, of a mixing activity on the bottom which makes this where the current speed is assumed to decrease as a layer dynamically different from the upper layer. logarithmic function to reach zero on the bottom; this theory supposes that shear is constant in the layer and The bottom mixed layer is usually associated with an intense particle resuspension whose vertical extent may that the Coriolis forces may be neglected compared with viscosity. Above this layer, the Coriolis forces sometimes be much larger than that of the BML itself. cannot be neglected and the Ekman theory is relevant; This layer above the bottom, where the suspended particle concentration is higher due to the presence of the current describes an "Ekman spiral" with its cha­ racteristic veering. In the oceans, ali the Ekman layers the bottom, is the "bottom nepheloid layer" (BNL). are turbulent. The logarithmic layer is the one-tenth As for the BML, it may result from the superposition deepest part of the Ekman layer. In deep oceans, the of locally resuspended particles and advected particles. The different aspects of nepheloid layers are reviewed order of the Ekman layer thickness is 10 rn and that by McCave (1986). of the logarithmic layer is 1 rn (for a velocity of 4 cmjs and a friction velocity u* of 0.2 cm/s). The theoretical It was with the aim of describing this environment that difficulty is to link these two layers mathematically. a CTD survey (with nephelometry) was planned during It is more difficult to observe these layers in the oceans, the moorings recovery cruise. particularly in deep oceans. In fact, the logarithmic The moorings were launched during the profile has been observed, but the existence of an "EDYLOC 81" cruise, in May 1981, aboard the R/V Ekman layer with its associated veering bas been less Le Noroît and recovered in April 1982 during weil established (Bowden, 1978). This is probably due . "EDYLOC 82" aboard the same ship. to the non-stationary currents (tidal oscillations) To exclude local topographie effects, the region selected encountered in the areas of observations. In the loga­ was a deep abyssal area (4780 rn) over the Porcupine rithmic layer, the time scale (of the order of zfu* which abyssal plain around 47°N, 14°30W, where the Tourbil­ is around 10 minutes) is small compared with the tidal lon experiment took place in 1979 (Fig. 1). period which allows the establishment of the loga­ The moorings array was designed as a cross, the rithmic profile. The time scale of the Ekman layer is branches of which were respectively parallel and normal of the order of the inertial period which is comparable to the continental margin. To obtain a maximum of to the tidal periods. Then, the tidal or inertial oscilla­ mooring pairs to compute the spatial correlations, the tions disturb the establishment of the Ekman layer. moorings were separated with increasing distances, the In our experiment, the deepest level (10 rn off the smallest being 11 km and the largest 73 km. A similar bottom) is quite surely above the logarithmic layer but experiment was conducted elsewhere during the same could be in the upper level of the Ekman layer if it period by I.O.S. (Institute of Oceanographie Science, exists. Comparison with the 4 000 rn results will allow UK), the results of which (Saunders, 1983) will be us to determine whether this is the case. compared with ours. 150 DEEP LAYER VARIABILITY IN THE EASTERN NORTH ATLANTIC Each mooring carried two Aanderaa current-meters, This deviee (Fig. 2) has been developed at IFREMER the deeper at 10 rn off the bottom and the shallower by Gouillou as a sensor dependent on the CTD both at 4000 rn depth, i. e. around 800 m off the bottom. for its power supply and its data acquisition system; This level was chosen as representative of the ocean its measurements are recorded simultaneously to those interior and will be used in a comparison with the near­ of temperature, salinity and oxygen.
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